This invention relates in general to a speed control apparatus for an alternating current motor driving an elevator car, and more particularly to such a three phase induction motor which is operated in three phase running, single phase running, and direct current braking modes.
A speed control apparatus for an induction motor operated in the three modes described above is disclosed in laid open Japanese patent application publication number JP No. 49-39,722, wherein one phase of the motor is controlled by means of two thyristors and two diodes in an acceleration mode, and direct current braking is performed by means of contacts changing over the circuitry in a decelerating mode. A similar apparatus is also disclosed in U.S. Pat. No. 4,083,431.
Japanese laid open utility model application publication number JP No. 52-169,262 discloses an arrangement wherein two phases of the motor are controlled by four thyristors in an acceleration mode, and direct current braking is again performed by means of contacts changing over the circuitry.
By way of a more specific example, a conventional speed control apparatus of the general type mentioned above is shown in FIGS. 1 to 3, wherein an elevator car 1 is connected to a counterweight 2 by a cable 3 wound around a sheave 4 driven by a three phase induction motor 5. The three motor phases are connected to terminals R, S and T of a three phase alternating current electric power source.
Contacts 6a, 6b of an electromagnetic switch or relay (not shown) which close when the car goes up and contacts 7a, 7b of a similar switch (not shown) which close when the car goes down are respectively inserted in the R and S phases of the motor. An electromagnetic switch contact 8 which closes when the car starts and opens when the car reaches a retarding command point as will be described later is connected to the T phase of the motor.
A thyristor or SCR 9 is connected in series between the R phase terminal and contact 6a, and ditto for a thyristor or SCR 10 between the R phase terminal and contact 6b, a thyristor or SCR 11 between the S phase terminal and contact 6a, and a thyristor or SCR 12 between the S phase terminal and contact 6b. Contacts 13a, 13b of an electromagnetic switch (not shown) which are closed when the car starts and opened after it reaches the retarding command point are respectively connected both across a series circuit comprising thyristors 9, 10 and across a series circuit comprising thyristors 11, 12. Contacts 14a, 14b of a braking switch (not shown) which are opened when the car starts and closed after it reaches the retarding command point are respectively inserted both between the thyristors 10, 12 and between the thyristors 9, 11.
A pilot or tachometer generator 15 which generates an actual motor speed signal Vt is directly connected to the motor 5. The output of this generator is fed to an adder 17 together with a speed command signal Vp from a generating circuit 16. The adder 17 generates a speed deviation or difference signal Ve which is fed to a firing circuit 18, which in turn generates firing signals for the thyristors 9-12.
In operation, assume that the car is going up under a heavy load. This condition corresponds to FIG. 3(a), which shows the states of contacts 8, 13a, 13b, 14a and 14b, and the running pattern. When a start signal is given and the contacts 7a, 7b are open, the contacts 6a, 6b, 8, 13a and 13b are closed while the contacts 14a, 14b are opened. This connects the thyristors 9, 10 in a parallel, reverse polarity circuit in the R phase as shown in FIG. 2(a), and similarly the thyristors 11, 12 in the S phase.
The speed command circuit 16 generates an accelerating command signal whose magnitude increases as a function of time while the car is accelerated, a constant speed command signal while the car runs at a constant speed, and a decelerating command signal whose magnitude decreases as a function of the position of the car while it decelerates, as shown in FIG. 3(a).
The firing circuit 18 generates firing signals for the thyristors to implement their phase control corresponding to the speed deviation signal Ve.
Thus, the motor 5 is controlled by way of a primary voltage controlling method and its torque is adjusted over a range from zero torque to full three phase torque.
When the car reaches the decelerating command point A in FIG. 3(a), contact 8 is opened which disconnects the T phase terminal of the power source so that only single phase power is supplied to the motor as shown in FIG. 2(b). This single phase running mode is transitionally employed because changing from a three phase mode directly to an open phase or D.C. braking mode would cause a rapid torque decrease and result in passenger discomfort.
The single phase running reduces the speed of the car, and when the actual speed signal V.sub.t1 exceeds the speed command signal Vp, the polarity of the deviation signal Ve changes. This in turn causes the contacts 13a, 13b to open and the contact 14a, 14b to close, whereby the thyristors form a single phase full-wave rectifier as shown in FIG. 2(c). This supplies a direct current I to the motor 5, which effects D.C. braking.
The conditions when the car is going up under a light load are shown in FIG. 3(b). In this case the car starts up with the three phase circuit shown in FIG. 2(a). Before it reaches the retarding command point A in FIG. 3(b), however, the actual speed signal V.sub.t2 exceeds the speed command signal Vp. Consequently, when the car reaches point A the contacts 8, 13a and 13b are opened, and the contacts 14a and 14b are closed. The operation of the motor is thus shifted directly from a three phase to a D.C. braking mode.
When the car is going down, the light passenger load condition corresponds to FIG. 3(a) and the heavy passenger load condition corresponds to FIG. 3(b).
In the apparatus shown in FIG. 1, the thyristors 9-12 functioning as the control elements for power running and for torque retardation are selectively switched by the contacts 13a, 13b and 14a, 14b. These electromagnetic contacts are not closed simultaneously, however, and thus there is some time delay in shifting the operation of the motor 5 from the A.C. running mode to the D.C. braking mode. This results in the torque generated in the motor being discontinuous or interrupted, which has undesirable effects on the control system. Such switching also accelerates the erosion or consumption of the contacts by reason of their repeated opening and closing, which requires considerable and costly maintenance work. Finally, such mechanical switch contacts are disruptively noisy during operation.